Last month I encounterd an oil-burner furnace, one that was itself a conversion of an older coal burner. It occurred to me that I am of the last generation to actually remember coal-fired furnaces, so I thought a little 'primer' was in order for those who do service work.

First, an overview:

If you look closely, just above the blower you can see doors for loading coal, as well as clearing out the 'clinkers.'

The blower has another component attached to it. That rectangular part, under the blower and to the left, is the ingnition transformer. Carefull - these generate 10,000 volts! The black coupling on the wiring actually IS an electrical coupling; electrical hardware once had a glossy black finish.

Now, if you followed those wires, one place they would lead is to this gizmo, mounted on one of the ducts:

Called a 'stack controller,' this is what tells the furnace to fire. The loose wires you see are the thermostat leads.

What I actually found in this basement was 'does it all circuit' -> blower assembly -> stack controller -> disconnect -> neutral. This lead to two undesireable things: 1) The basement light was out when I turned off the furnace power; and,2) The HVAC guy got shocked even though the disconnect was opened; everything was still 'hot,' the result of having the disco LAST, rather than first.

How did this happen? Well, the original sparky wired the knob & tube up that way (white wire as 'hot'), and the various homeowner additions only made it worse. Here's the splice:

Finally, here's the basement lighting I was deprived of with the furnace power off:

You can glean much information from a picture. The make 'RAY' is unknown to me, a european, but the principle of the pressure-jet burner is universal. A gear pump draws oil from a tank. This passes to a pressure relief valve, usually set at about 100psi. Some oil is directed to a nozzle jet, where it is broken up into a spray-mist and ignited by a spark of 5-10,000v between ignition tips. The flame is formed in a 'retention head' to keep it attached to the burner and not get blown out by the high flame velocity. Excess oil [ up to 90% of the total drawn ] travels back to tank - the jet size and the pump pressure control the amount of oil burned. This is usually marked on the nozzle in US gallons/hr at the [standard] 100psi. The two line-fuel arrangement shown here is recommended if the tank is lower than the burner, to prevent oil cavitation. The motor, usually only about 100W, drives the gearpump and also an axial-blower which provides air via a variable flap, all combustion air being under pressure and adjusted to a get clean efficient burn. These burners can handle a variety of oils and kerosenes, but usually burn diesel of some sort, as it's cheaper.

Around here they were commonly added to the loading door of old woodfired boilers, hot-air furnaces being a rarity, and seemed to work quite well, if a bit noisy.

Underslung from this setup in the black box is probably the 'controller'. This usually employs a flame monitoring photo-cell device to provide 'flame out' safety before during and after burn, locking out the burner if 'unsheduled' burns occur! This is handy, as it precludes spraying oil into a flue fire! It also switches out the ignition arcs after a delay of about 15-30 seconds to conserve the igniter tips, and sometimes a pre-purge start delay to get the pump line up to pressure before firing- this saves fuel by ensuring a good spray at start. Recently electronic versions have appeared, displacing the mechanical-switched versions of yore, designed specifically to stop Alan building specials from junked units displaced by woodpellet burners!

I notice the RAY is standing on a Cast Iron tripod adjustable for height - marketed as an add on for solid fuel conversions perhaps?

Reno, I think the wiring may also be incorrect, [apart from the shock hazard!!] if shutdown also turns off the light! The duct stat should shut down the burn as 'furnace hot enough' by signalling the controller. The hot air should continue to flow to the dwelling by the ducting blower. A stat or programmer in the house should initiate 'call for heat' and 'call satisfied' signals to the controller and simultaneously control the ducted air blower to turn on/off the burn and the flow of hot air. Burns should be as a long duration as possible to use less fuel, as start ups are less efficient.

I might add my atavar shows an early model of my 'waste vegetable oil' burner, running on test at night to get a good flame pic [ and deemed unsuitable due to unburned oil in the flame lacquering the boiler innards!] The 'flame retention head' is just visible, glowing dull red hot. This is about 100,000 BTU / 30kW power, based on Danfoss parts.

I'm currently able to get a blue 40kw flame from WVO by thermally cracking the oil at 700F before the burner head, unit mothballed till the spring as other projects have taken priority, like 'her indoors' insisting on a kitchen! I'll post more then.

That is good details there John and yes I do remember early coal fired boiler / warm air furance those unit were huge and I did work on one steam boiler { 15 PSIG system } it took a while to get the steam up but once you got there it stay there for very long time and it last pretty long time before have to refired again.

Those common resdentail / light commercal- industrail burners you can able change the firing rate { size } by changing the nozzle size and adjust the air door a little.

Yeah they can burn almost any fuel it go thru but not gasoline { too fast for combustion chamber } they can burn #1, #2 Diesel fuel / Heating fuel , Keronse , Jet fuel , some case old engine lube oil { if preheated yeah it will burn pretty good }.

Alan, you are correct ... our codes have required furnaces to have their own dedicated circuit for decades. That is, the circuit needs to supply nothing but the furnace and associated equipment (for example, a hunidifier or air cleaner). Or, with proper switching, an air conditioner can use the same source of power.

This house, though ....The panel was on the outside of the house, and the circuit ran up the wall to the attic. Where else it went on it's journey through the house, back to the basement, is anyone's guess. All I can say for certain is that the light, and several other things, went out when the heating guy shorted the wires to ground.

A reminder ... he didn't do that deliberately ... rather, he opened the disconnect, adn shorted the wires while attempting to remove the stack controller. They were still hot, as the disconnect was on the "neutral" side of the circuit.

You often find the neutral and hot reversed in knob and tube installations, as the insulation is often all the same color: black. This house was the exception; there were cloth-covered wires in black, red, green, and white.

Lest one become confused ... remember, Knob & tube wiring pre-dates then use of green wire for grounding. In such a house, the green is very likely to be hot! When I had sorted out the wiring, everthing was hot ... EXCEPT the black, which was the neutral!

I 'solved' this one by running a new, dedicated, circuit to the disconnect, and re-wiring the furnace. Now, the furnace just may be the only code compliant circuit in the house.

If I may ask a dumb question, coming from a country where residential central heating is not common, (certainly not this oil fired kind), what stops the flames and/or carbon monoxide going into the ducts? Is there some kind of heat exchanger? If so, what if it leaks?In Australia, oil heaters were popular from the 1960's up until the mid 70's, but they were installed in the living room, like a gas heater or enclosed fire place. The oil seemed to be more like kerosene, and I discovered it burnt just as well in hurricane lamps. There was a tank mounted on the side of the house that would need filling about once a year. Used to be a common sight around suburbia to see an oil truck parked outside someone's house with its black hose trailing down the driveway.We had a late 60's model in the first house I lived in, which burst into flames in the middle of the night. It had a burner much like a gas ring, and you opened the door to light it with a match. It was replaced by a more modern version that looked more like a gas heater, with ceramic panels to radiate the heat from the burner underneath. Ignition was from a heating element; after a few minutes it would suddenly burst into flame.

The other thing I forgot to mention here that some of the old furance some are gravity curicationed { sans force air blower } as warm air rise the cool air sink that how it work on old furnce but one major gotcha what many oldtimers know do not overfire {overfueled} them.